The present invention relates to wireless digital communications systems. More particularly, the present invention relates to code-division multiple access (CDMA) communication systems where a subscriber unit and a base station communicate with each other using different bandwidths.
CDMA systems provide an efficient use of the limited bandwidth of the RF spectrum, thereby permitting a greater amount of information transmission with less signal distortion than communications systems using other techniques, such as time division multiple access and frequency division multiple access.
In a CDMA communication system, an information signal at the transmitter is mixed with a pseudo-random spreading code which spreads the information across the entire bandwidth employed by the system. The spread signal is upconverted to an RF signal for transmission. A receiver, identified by the same pseudo-random code, downconverts the transmitted spread-spectrum signal and mixes the downconverted signal with the same pseudo-random code that was used to spread the original information signal to reproduce the original information signal.
A prior art CDMA communication system is shown in FIG. 1. The communication system has a plurality of base stations 201, 202, . . . 20n connected together through land lines via a local public switched telephone network (PSTN) or by a wireless link. Each base station 201, 202, . . . 20n communicates using spread spectrum CDMA transmissions with mobile and field subscriber unit stations 221, 222, . . . 22n located within its cellular area.
In a typical prior art CDMA system, downlink bandwidth which is used by the base station to transmit to the subscriber unit, is the same as uplink bandwidth which is used by the subscriber unit to transmit to the base station. Symmetrical allocation of bandwidth is appropriate where the uplink and downlink data volumes are roughly equivalent, as in the case of voice communication. However, in some communication scenarios, allocation of equal bandwidth to uplink and downlink transmission is an inefficient use of the limited RF spectrum available to a wireless communication provider. For example, an individual using the Internet generally transmits a limited amount of data, which might include Internet addresses, search terms and typed responses to queries. In contrast, an Internet server generally responds to a user's request and the user receives large amounts of text, graphics and other forms of data. In this case, providing a larger downlink bandwidth for the transmission link from the base station to the subscriber and a smaller uplink bandwidth for transmission link from the subscriber to the base station permits a more efficient use of the total bandwidth allocated to the communication provider. Though the same total bandwidth is used in an asymmetrical bandwidth communication as in one where the uplink and downlink bandwidths are the same, in an asymmetrical communication the higher use downlink channel can send data more quickly by occupying a larger bandwidth without sacrificing performance of the uplink channel, which sends its limited quantity of data at a lower rate.
In a typical prior art CDMA system, a subscriber unit generates a pseudo-random spreading sequence which is repeated every 29,877,120 clock cycles. A full sequence is known in the art as an epoch. A prior art system 200 for generating a pseudo-random sequence is shown in FIG. 2. A data clock 202 is fed to a first code generator 204 which creates a pseudo-random sequence of 233,415 chips, and to a second code generator 206 which creates a pseudo-random sequence of 128 chips. The outputs of these two generators are combined to produce a pseudo-random sequence of 233,415×128, which is 29,877,120 chips long. At the end of the sequence, the code generators 204, 206 restart the code from the beginning of the sequence.
When a subscriber unit first begins to transmit, its pseudo-random sequence is free-running. Its epoch is not synchronized with the epoch of the pseudo-random sequence being generated at the base station. Accordingly, the base station must search for the start of the pseudo-random code of the subscriber unit, which is a time-consuming process.
In a prior art system as described, the use of different bandwidths means that the pseudo-random spreading sequence is clocked at different rates for the uplink and the downlink. FIG. 3 shows epoch starting points for an uplink 120 and a downlink 100, where the downlink clock rate is twice that of the uplink. As shown, the starting points for the uplink epoch, 122 and 124, are aligned with every other downlink epoch starting point, 102 and 106. This creates an ambiguity in that the subscriber unit, which is attempting to decode downlink data, is unaware of the start of the downlink epoch. For a downlink transmission that begins in the uplink epoch starting at 122, the downlink starting point could be either 102 or 104. This ambiguity causes the subscriber unit to search the entire sequence to find the starting point. This process consumes an unacceptably long amount of time, thus rendering impractical the use of asymmetrical bandwidths.
Consequently, the need exists for a CDMA system where the pseudo-random spreading codes of the uplink and downlink remain synchronous when the uplink bandwidth differs from the downlink bandwidth.